Inter-office communication system



Feb. 8, 1938. J, D s 2,107,817

INTER-OFFICE COMMUNI CATION SYSTEM Filed Jan. 29, 1955 -3 Sheets-Sheet l Feb. 8, 1938. J. DAVIS INTER-OFFICE COMMUNICATION SYSTEM Filed Jan. 29, 1956 3 Sheets-Sheet 2 jay Z15;

Feb. 8, 1938. J. l. DAVIS 2,107,817

INTER-OFFICE COMMUNICATION SYSTEM Filed Jan. 29, 1936 s Sheets-Sheet s ATTORNEY 3.

Patented Feb. 8, 193a PATENT OFFICE INTER-OFFICE COMMUNICATION SYSTE Jack I. Davis, Chicago, Ill., assignor to Sidney Ludwig, Chicago, Ill.

Application January 29,1936, Serial No. 61,418

4 Claims.

This invention relates to an inter-office communication system, and more particularly to a system wherein a plurality of units in different parts of the ofiice are connected by a cable.

One feature of this invention is that it results in better quality of received speaking; another feature of this invention is that it avoids crosstalk between different lines in the same cable; yet another feature of this invention is that it enables a visual indication of the party calling another, over the same cable wire as the voice transmission; still another featureof this invention is its simplicity of installation; another feature of this invention is the ease with which additional units may be connected to those already installed; a further feature of this invention is that it may be operated entirely from the commercial source of alternating current; still another feature of this invention is that it may be used as a conference communication system; other features and advantages of this invention will be apparent from the following specification and the drawings, in which- Fig. l is a schematic diagram showing such a 25 system, one unit being shown in detail; Fig. 2

- illustrates the manner of connecting the various units by cable; Fig. 3 is a diagrammatic view of one ofthe three-way socket-elements used for connection; and Fig. 4 is a front elevation of a 30 preferred form of unit.

Modern inter-office communication systems of the type designed for use by executives particularly are subject to many disadvantages. The

majority of these systems comprise a unit for a 35 desk including a microphone and a speaker, and

having switch means for connecting it with any chosen unit in the system. As now designed, most of these transmit the microphone output through the cable or other connecting network to the 40 other unit, and shielded connections are necessary to prevent cross-talk and other interference.

Most of these systems also necessitate additional wires to transmit current to operate an indicating device. The installations are expensive, and

45 diflicult to change or to provide for the addition of other units.

The present invention comprises an inter-office communication system having a plurality of desk units each having a microphone, a vacuum tube amplifier for the output of the microphone, a

i speaker, an indicating device, switches, and a power pack to enable operation from commercial alternating current. A single cable containing only one more wire than the number of units is 55 connected to each of the units by a feeder cable.

The main cable and feeder cables are of low impedance, and the amplified output of the microphone is transmitted through the cable, the line impedance being substantially reached at each end of the transformer. 5 Referring more particularly to Fig. 1, each unit, adapted to be placed on a desk, contains a microphone III or other means for translating sound waves into electrical energy, an amplifying sys tem for the output of the microphone, indicated here in general by the numeral H, a speaker indicated in' general as I2, a power pack iii, an indicator l4, and the necessary switches. A feeder cable l5 connects to the main cable i6, to which are also connected other desk units l1, I8, and

Hi. It is to be understood that the number of wires contained in the main cable and feeder cable are covered by the number of units which it is desired to use, in each case being one more than that number. 2 The microphone Iii is preferably of the carbon button type, although it may be a ribbon type or any other in general use. Its output is passed through the primary of the transformer 20, power for it being supplied from the power packi3 through the line 2i, a filter system comprising resistors 22 and 23, radio frequency choke coil 24, and condenser 25. The voltage developed in the secondary of the transformer 20 is applied through the volume control 26 to the grid of the 30 triode tube 21. The cathode 28 of this tube is grounded through the bias resistor 29, shunted by the condenser 30. The heater 3| is supplied with current by the secondary 32 of the power pack. The plate 33 of this tube is connected to the grid 34 of the pentode tube 35 through a resistance coupling network comprising resistors 36, 31, 38, and 39 and coupling'condensers 40, ll and 42. The cathode 43 of the pentode tube 35 also has a. biasing resistor 44 in series therewith, shunted by a condenser 45. The heater 56 is also supplied with current from the secondary 32 in the power pack l3. It also has connected thereto the suppressor or grid 46. The output of the tube from the plate 46 is passed through the switch 48 and shunted to ground by the condenser 49, and the primary of the transformer 50. This transformer is inserted in order to match the output impedance of the tube substantially with the line or cable impedance, and the switch 43 is pro- 50 vided so that when it is desired to talk to more than one of the other units, the switch may be changed in order to match the output impedance more nearly to the impedance of several units in parallel.

The output of the line-matching transformer 50 is connected to a bus wire to which are connected the contact points 52 of a number of switches, of which three are here shownas 53, 54, and 55. Direct current power, supplied by the power pack I3, is drawn from the lead 2| through the resistor 58 and the resistors, to the switch points 60 of the switches 53, 54 and 55. The points ii are the 011 positions of the switches. The resistors 59 would be the same for each of the switches in a given unit, but would be different in each-unit. Thus the direct current flowing from the unit when any switch is on the point 60 would be diiferent for each unit. BI is the off position for each switch.

The input to each unit, transmitted throughthe cable from some other unit, is through the wire or lead 62 which branches it in one case to the indicator H, in series with the operating coil 63 of a relay having an armature 64 making contact with the point 65, and in the other case to the armature 64. This armature is, normally maintained in contact with the point 55 by a spring 66. The voice coil 61 of the speaker I 2 is connected from 65 to ground. Thus when the armature G4 is contacting 65, the amplified output of. the microphone of another unit, transv mitted through the main and feeder cables, energizes the speaker and results in translation of the electric energy into audible waves.

The indicator II is essentially an ammeter, chosen with a range such that each of the resistors 59 heretofore mentioned causes the needle of the indicator to stop at a difierent position on the face thereof, as'may be more readily seen from Fig. 4. That is, if the operator of the unit shown in detail in Fig. 1 wishes to speak to the unit l8, he would first throw his switch 53 to the contact 60, which would send a direct current through the cable to the indicator in the unit I8. This direct current would energize the relay coil in that unit and cause the contacts to break, thus opening the circuit through the speaker voice coil 61. The indicator needle would swing to a position governed by the value of the resistor 59, and would in this case take the first position to show that unit No. 1 was calling. As soon as the unit l8 has answered, the operator of unit No. 1 would throw switch 53 on over to point 52, whereupon the spring connected to the relay armature would restore connection to the speakei of. unit 18, and communication would be carried on.

It is also contemplated that a small light or other indicating device might be used in series with the indicator II to more readily call attention to the fact that one is being called over the system.

Referring again to Fig. 1, the process willbe described when the operator of the unit 18 answers the unit shown diagrammatically in Fig. 1. When the operator of l8 was signalled by the closing of the switch53 the current travelled along the line or lead 85 between the contact points 86 and 81 of the plugs (through the set at signalling position and direct current is used it is prevented from passing through the relay coil 61 through the speaker l2 by blocking means here shown as a relay energizing coil 83 which opens the contacts between 64 and 65; the voice modulated alternating current from the amplifier of'the unit ill, on the other hand, passes through the operating coil 61 and energizes the speaker l2. It will be noted that the speaker in each unit has no connection with the amplifier of that unit; on the other hand, the operating current supplied to the speaker of a unit-qis in each case developed by the amplifier in another unit and transmitted through the low impedance cable to the speaker.

The power pack indicated in general as l3 comprises a transformer 68 and a full switch rectifying tube 59. The heater Ill is operated from the secondary 51. The field coil H of the speaker I2 is inserted in the output of the tube 69 as a, load, and is shunted to ground by condensers l2 and 13 to make a filter network for the output of the rectifier. A switch 14 is provided to control the alternating current inputto the transformer 68.

The cabling arrangement and inter-connections between units may be seen more clearly by reference to Fig. 5, which illustrates schematically this invention as embodied in an interoflice communication system comprising four units, each of these units being identical with that shown in detailin Fig. 1, the cabling corresponding to that shown in Figs. 2 and 3. Each of the four units has a feeder cable connecting.

sockets each feeder cable from each unit may be attached to the desired wires in the main cable without splicing or soldering and similar additional units may be added, up to the limit of the number of. wires in the main cable.

Referring again to Fig. 5, the operative connections and circuits between units will be more fully described. Inasmuch as the schematic illustration in this figure shows the device embodied only in connection with four units rather than with the five unit system shown in Fig. 1, the parts will be indicated by new reference numerals. It will be understood, of course, that each unit is similar to that described in complete detail and illustrated in Fig. 1, in that it contains a microphone, an amplifier, a speaker, an indicator, and control switches. of more units would merely necessitate the addition of further switches and further wires in the cable, the switches always being one less than the number of units and the wires in the cable one more than the number of units.

The left hand unit illustrated in Fig. 5, and here referred to as llll, is shown connected to the third unit I03. That is, unit IM has its switch marked 3 set in the talking position; and unit I03 has its switch marked I set in the talking position. Thus any sound input to the mi-' The addition through the switch I and the wire III to the speaker 2 of the first unit. The return circuit in each case is made through the ground wire H3 common to all of the units. Circuits are thus formed with a minimum of wires ennbling units I and 3 to talk to each other, the

circuits being distinct so that the users may talk simultaneously, break in, or in any other way they desire.

Similarly, units I02 and I04 are shown with their switches in such position that they may talk -to each other at the same time that conversation is going on between units iii and I03 with no interference. Switch No. 4 of. the second unit is set in the talking position, so that a circuit is completed from its amplifier through the wire 4 to the speaker of unit 104. This last mentioned unit has its second switch in talking position, so that a circuit from its amplifier is completed to the speaker of unit I02 through the wire H5.

As heretofore described, not only may pairs of units talk independently of each other, but any number of units may sit in what might be called conference arrangement. That is, it would only be necessary for unit ll to throw all three of his switches to the talking position to supply the output of. his amplifier to each of the other. speakers. Thus the user of the first unit could speak into microphone I05 and be heard simultaneously by the users of each of the other three units. If desired, any of the other units could throw a multiplicity of their switches in the talking position and answer the first speaker, there the last user's voice being reproduced in such speakers as he chooses by the throwing of switches.

Referring again to Fig. 1, it will be noted that the operating coil 63 of the relay is in parallel with the voice coil 51 of the speaker i2. This condition does not prevent operation of the speaker, however, since the impedance of a voice coil, usually only a few turns of wire, is very low compared to the impedance of the relay winding to currents of.,voice frequency. While such relay winding has a relatively low current resistance, and thus enables easy operation of the indicator 14, it has an alternating current impedance many times that of the voice coil, and thus the major portion of power of an alternating current would be developed in the speaker rather than being shunted out through this relay winding The values of the resistors and condensers heretofore described are chosen in accordance with the particular tubes and circuit employed,

and may vary considerably. In the particular embodiment of this invention disclosed in Fig. 1, tubes 21 and 35 are models F and F respectively, and the rectifier 69 is a Z. With these tubes, and the particular circuit disclosed, the following values were used: the volume control 26 was 200,000 ohms; resistors 22 and 23, 20,000 ohms and 10,000 ohms respectively; resistor 29, 2,500

ohms; resistor 44, 400 ohms; resistors 36, 31, 38

necting another unit to the main cable by a feeder. Fig. 3 is a schematic diagram of one of the socket-elements ll, and-illustrates how the sockets are connected. It will be noted that each socket is provided, in addition to the holes I! into which corresponding pins on the plugs 00 fit, with a recess ll adapted to receive a corresponding key on the plugs 00. Thus. the various plugs, whether on the sections or portions of the main cable or on the feeder cables, must always be inserted in the proper way in the socketmembers II. 'When it is desired to install an inter-ofllce system of the type described herein, it is not necessary to buy and install at one time the full number of units which it is contemplated using. The main cable is chosen to have a number of wires one more than the number of units which it is contemplated will eventually be installed. The system may then be started with only two or three units, if that be desired. Additional units, up to the limit of the capacity of the cable, can be added at any time by merely purchasing additional sections of. cable and additional socket elements and plugging them into the already operating portion of the system.

As was heretofore mentioned, this system may beused to speak to one other unit or several other units at once. When only one unit is being spoken to, the switch 48 would be in the position illustrated in Fig. 1; when speaking to several other units at once, the switch would be thrown to its other position in order to more nearly match the impedance of the speakers of the other units in parallel. In this connection it is to be understood that speaker is used to cover the actual sound-producing device and its associated parts, including, for example, a matching transformer. Inasmuch as the output of the transformer 50 is placed on the bus bar or bus wire 5|, any number of switches may have the output directed therethrough at once, up to the limit of the number of switches on the unit. When the word "number is hereafter used in the claims, it will be understood that it means any number of units from one up to the total number of units in the system.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims, in which it is my intention to claim all novelty inherent in the invention as broadly as permissible, in view of the prior art.

What I claim as new and desire to obtain by Letters Patent is:

1. An inter-omce communication system of the character described, including: a plurality of units, each unit comprising a speaker, a microphone, tube means for amplifying the output of said microphone, and an indicator; a cable connecting said units, said cable having therein only one more wire than the number of units; a source of direct current associated with each of said units; and a relay breaking the circuit to said speaker when said indicator is being energized by direct current.

2. An inter-office communication system of the character described, including: a plurality of units, each unit comprising a microphone, tube means directly connected thereto for amplifying the output of said microphone, a transformer for coupling said tube to a low impedance output circuit, a speaker, and switch means; and a cable connecting said units, said cable and switch means being so constructed and arranged that said cable may be connected to the transformer of each unit to form a part of thelow impedance output circuit for transferring energy to any number of the speakers 01' the other of said units.

3. A unit adapted to be connected to any num-- is adapted to energize the speakers of any number of said other units.

4. A unit adapted to be connected to any numberoi' other similar units by a cable including only one more wire than the number 01' units to form an inter-oflice communication system of the character described, including: a plurality of units, each unit comprising a microphone, tube means directly connected thereto for amplifying the outputof saidmicrophone; switch means;- .an indicator; and means permitting energization or said indicator without energization 0! said speaker, through the same pair of wires in said cable effecting energization oi. said speaker, as

a result of the operation of the switch means of 1 any of said other units.

- JACK I. DAVIS. 

